1. Field of the Invention
The present invention relates to an anisotropic conductive adhesive, and more particularly, to an anisotropic conductive adhesive which is used to connect a drive integrated circuit (IC) to a substrate on which electrodes are formed to a fine pitch.
2. Description of the Related Art
Methods of mounting a drive IC on a liquid crystal display (LCD) panel can be classified into a wire-bonding method, a tape automated bonding (TAB) method, a chip-on-glass (COG) method, and the like. In the wire-bonding method, a drive IC is bonded to electrodes on a LCD panel via conductive wires. In the TAB method, a drive IC is mounted on electrodes on a LCD panel using a base film. In the COG method, a drive IC is mounted directly on a LCD panel using a predetermined adhesive. Among theses methods, the COG method has advantages in that an area for mounting a drive IC over a LCD panel can be minimized and the costs for mounting the drive IC over the LCD panel can also be minimized. Thus, the field of use of the COG method can extend. In general, when a drive IC is connected to a LCD panel by the COG method, an anisotropic conductive adhesive is used to electrically connect electrodes of the drive IC to electrodes on the LCD panel.
Currently, LCD panels tend to be large in size and electrodes tend to be fine in order to satisfy the need for storing a large capacity of display information, displaying high-quality information, and so forth. Due to this, the width and thickness of a signal line on the LCD decreases, and a pitch or a gap between electrodes becomes reduced with a reduction in the area of the electrodes or bumps for electrically connecting the drive IC to the LCD panel. In practice, the pitch between the bumps or the electrodes mounted on a PC monitor or a portable telephone screen using the COG method is about 100 μm, and a gap between electrodes is about 50 μm. However, the widths of pitch and the gap is becoming gradually reduced.
Accordingly, an anisotropic conductive adhesive, which is capable of maintaining an adhesion state between a LCD panel and a drive IC with a stronger adhesive strength as well as electrically connecting a larger number of electrodes in a limited space, is required.
However, a prior art anisotropic conductive adhesive is limited in connecting a drive IC to a substrate, on which electrodes are formed to a fine pitch. In other words, when a drive IC is connected to a substrate, on which electrodes are formed to a fine pitch, using the prior art anisotropic conductive adhesive by the COG method or a chip-on-film (COF) method, a short-circuit may easily occur between adjacent bumps or adjacent electrodes due to a gap between the adjacent bumps or the adjacent electrodes. The reason is why a resin constituting an anisotropic conductive adhesive has flowability due to heat applied in an adhesion process using the anisotropic conductive adhesive, conductive particles contained in the anisotropic conductive adhesive flow into gaps between adjacent bumps or adjacent electrodes, and the adjacent bumps or the adjacent electrodes are short-circuited by the conductive particles.
FIGS. 1A and 1B are cross-sectional views explaining a process of connecting a drive IC 20 to a flexible substrate 10 made of polyimide by a COF packaging method using an anisotropic conductive adhesive according to the prior art. Here, FIG. 1A shows the state before an adhesion process and FIG. 1B shows the state after the adhesion process.
Referring to FIGS. 1A and 1B, electrodes 12, which are formed on the substrate 10 of a metal such as Cu or Al, and bumps 22 on the drive IC 20 each have a width WB of about 28 μm and are each formed to a pitch P of about 38 μm. Thus, gaps, each having a width WG of about 10 μm, are formed between the electrodes 12 and between the bumps 22. Also, each of the electrodes 12 are formed on the substrate 10 to a height HE of about 15 μm while each of the bumps 22 are formed on an Al electrode (not shown) of the drive IC 20 to a height HB of about 20 μm.
In a process of adhering the drive IC 20 to the substrate 10, a resin and conductive particles 32 of an anisotropic conductive adhesive 30 between the bumps 22 and the electrodes 12 are supplied with heat and pressure. During this process, the viscosity of the anisotropic conductive adhesive 30 becomes lower, and the resin and conductive particles 32 flow into the gaps between the bumps 22 or between the electrodes 12. As a result, as shown in FIG. 1B, after the adhesion process, the number of conductive particles 32 between the bumps 22 and the electrodes 12 is on average smaller than the number of conductive particles 32 in the gaps between the bumps 22 or the gaps between the electrodes 12. Thus, it is impossible to keep the resistance between the electrodes 12 and the bumps 22 low, and the conductive particles 32 are electrically connected to each other between the bumps 22 or between the electrodes 12, thereby increasing the risk of short-circuits occurring. Here, the larger the conductive particles 32 and the higher the number of conductive particles 32, the greater the possibility of short-circuits occurring.
After bonding using an anisotropic conductive adhesive, the conductive particles contribute to the connection between bumps and electrodes in order to obtain proper electrical conductivity between the bumps and the electrodes. For this, a sufficient number of conductive particles need to be contained in the anisotropic conductive adhesive. However, if too many conductive particles are in the anisotropic conductive adhesive, the electrical resistance may be lowered and the possibility of short-circuits occurring becomes higher. In contrast, if there are too few conductive particles, the number of conductive particles contributing to the electrical connection between electrodes is reduced, which may result in an increase in the resistance between the electrodes. This phenomenon occurs because conductive particles, which have to be on the electrodes during the adhesion process, flow into gaps between adjacent bumps or adjacent electrodes. Thus, if the anisotropic conductive adhesive contains more than the proper number of conductive particles, a short-circuiting risk is increased, manufacturing costs are increased, and an excess of conductive particles exists in the gaps between the bumps or the electrodes. As a result, the adhesive strength of the anisotropic conductive adhesive deteriorates. In order to prevent the conductive particles from flowing into the gaps between the bumps or the electrodes, a method of increasing the viscosity of the anisotropic conductive adhesive can be considered. However, in such a method, smooth adhesion cannot be achieved and voids are formed in the gaps, thereby deteriorating the mechanical strength of the anisotropic conductive adhesive as well as the adhesive strength.